This continuing effort is responding to the increasing demand for a more precise measurement of relevant genomic information in any viral infection. The knowledge of the presence of a specific viral gene will help in identifying the infectious agent. However, to assess the stage of a disease, to evaluate the efficacy of a treatment, or to determine the value of a predictor of the progression of a disease, a more precise and quantitative analysis of the specific gene would be required. These previously research-oriented questions can now begin to be answered in routine clinical laboratories with the advanced technology of molecular biology, such as polymerase chain reaction (PCR) and the sequencing and mapping of restriction nuclease-digested fragments. We initiated developmental research in molecular diagnostic technology to meet our clinical study needs in hepatitis B virus, hepatitis C virus (HCV), and HIV infection. Whenever possible, we would improve the basic PCR technique to become a semi-quantitative procedure. During the last 2 years, we were able to use PCR as primary study tool for viral infection by a newly identified human hepatitis virus, HGV. We found that this virus was transmitted by blood transfusion: specific HGV RNA was identified in both recipients and paired donor sera. HGV can cause chronic infection with mild or no observed liver function abnormality. in the preliminary studies, the prevalence of HGV in blood donors was higher than that of HCV. We also initiated a project to construct an internal standard to be used in RT/PCR for determining HCV RNA. A project to study the unique specificity of the repairing enzyme Mut Y and its clinical application was also initiated during last year. Our future effort in this program will be to continue to evaluate new testing approaches, specifically, perfecting the quantitation procedures, and standardizing and applying molecular testing to blood products.